致命错误：调用未定义的方法ConnectDatabase :: query（）

Question

我创建了一个OOP-PHP脚本，我得到了关注消息，我不明白为什么???

致命错误：在第73行调用未定义的方法ConnectDatabase :: query（）....

我的PHP代码

<?php

class ConnectDatabase {

private $host = 'localhost';
private $username = 'root';
private $password = '';
private $database = 'database';
private $db_connection;
private $db_query;

//Connect Database.
  function __construct() {
  $this->open_db();
}

public function query($sql) {
 $this->db_query = query($sql, $this->db_connection);
}

public function open_db() {

$this->db_connection = new mysqli($this->host, $this->username, $this->password);

if (is_resource($this->db_connection)) {
  die("Error!");
 } else {
  $this->db_connection->select_db($this->database) or die('Error');
  }
 }
}

$dbConnection = new ConnectDatabase();

class GetContent {

 public function newContent() {
 global $dbConnection;
 $sql = "SELECT * FROM mytable";
 // ----- This Line make an Error ------
 $query = $dbConnection->query($sql);
 $found = $dbConnection->fetch_array($query);
 return $found;
 }
}

如果您有任何想法，请帮助。